California forecasters have warned that the atmospheric river is approaching

Another atmospheric river system has set its sights on California, raising significant concerns about flooding and structural damage as warm rain is expected to fall on top of the state’s record snowfall this week, forecasters say.

“A significant and very likely warm atmospheric river event could affect parts of northern or central California late Thursday and Saturday,” UCLA climate scientist Daniel Swain said during a briefing Monday.

Last week, the chances of such a system forming were around 20%. By Monday, the chances had increased to “7 or 8 out of 10, otherwise, for some degree of warm atmospheric river event,” Swain said. There could be one more storm this month.

Atmospheric rivers rise in mountain ranges and dehumidify them.

(Paul Duginsky/Los Angeles Times)

The forecast comes as California is in the midst of one of its wettest winters in a remarkably deep snowpack. A series of nine atmospheric river storms hit the state in early January, causing embankments, widespread flooding and almost two dozen deaths.

In recent weeks, severe winter storms have dumped fresh piles of powder in the Sierra Nevada and other areas, including the mountains of Los Angeles and San Bernardino counties, where some residents have been trapped behind layers of snow.

Officials said the gift made a dent in the state’s severe drought conditions and gave some hope to the filtered water supply after three bone-dry years. But it becomes a danger if heavy snow meets warm rain, which melts very quickly.

“We’re going to see rain on top of the snow, and at 2,000 feet to 4,000 feet, a lot of that snow is going to melt,” said Carlos Molina, a meteorologist with the National Weather Service in Hanford. California. “We’re basically going to lose a lot of the snow that fell from the previous storms. We’re looking at the potential for flooding.

In fact, the highest potential for flood-related damage is in low-elevation areas with unusually deep snowpack, Swain said. Smaller rivers and streams in those areas will see significant potential for flow problems, as in some urban areas — especially where storm drains are already clogged with snow.

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Above 5,000 or 6,000 feet, there can be problems, he said. Although the snow in such areas is probably too deep and too cold for the storm to melt, it becomes heavier as it absorbs more water. This can cause roof collapse and other structural problems.

“If you’re going to go out and try to clear some snow off vulnerable structures, do it,” Swain said. The state has already seen roof collapses due to snowfall. Including grocery store Avalanche provides critical material at the crestline.

Scott Rowe, a meteorologist with the weather service in Sacramento, said an incoming low-pressure system is forming in the north, but is expected to be coupled with “very warm” subtropical moisture from Hawaii. Such storms are sometimes referred to as Pineapple Express and are known to drop high humidity in California.

“We’re basically transitioning from one storm track to another where the origin of moisture is coming from a warmer, juicier place,” Rowe said.

While such a system is certain to arrive, its precise timing, location and impacts will become clearer as the week progresses, forecasters said.

The weather service for the Central Coast and San Francisco Bay Area said in its forecast, “Hold on, it’s going to be quite a weather ride.” Coastal areas could get up to 6 inches of rain Thursday morning through Saturday afternoon, and other parts of the region could get up to 2 inches, the agency said.

The weather service in Sacramento similarly warned of several inches of rain and high snow levels. It said there was a 40% chance of small rivers and streams rising, with the risk of road flooding especially on Friday.

In the central part of the state, areas above 8,000 feet — including Yosemite National Park — could see up to 6 feet of snow, while areas between about 5,000 and 8,000 feet could see up to 4 inches of rain. Up to 1½ inches of rain is possible in the central valley floor, and flooding is possible along the Merced River.

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While the storm’s impact is currently expected to hit northern and central California, southern California could also experience impacts including river flooding and creek flooding in Santa Barbara and San Luis Obispo counties, meteorologist David Gombert said. Oxnard Weather Service.

“The probabilities increase as you go north from Santa Barbara, but we don’t want to completely ignore L.A. and Ventura because there could be problems there,” he said.

More worrying is the possibility of more rain and snow in the mountains of San Bernardino County, where crews on Monday were still struggling to clear roads and help stranded people. Chances of increased precipitation there are expected to begin early Friday morning and continue through Saturday, said Elizabeth Schenck of the National Weather Service’s San Diego office, which covers the San Bernardino area.

Snow levels in the area could be as high as 8,000 or 9,000 feet, he said, “meaning that most of the precipitation will be rain,” including amounts up to 1½ inches.

“It’s not good because any rain that falls on the snow is going to accelerate the melting of the snow in that area, and with how much snow they’ve already gotten over the last week and a half, that’s a lot of snow equivalent to water, so it’s going to lead to a faster acceleration of the snowmelt.

While the coming storm may have moderate impacts, Swain said he fears it could prime the state for bigger problems. Forecasts show at least one more system could follow.

“If we get persistently warmer atmospheric rivers first, there’s the potential for more flooding,” he said. “There’s a 1 in 3 chance of that, so the odds aren’t low right now.”

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There are other factors that can increase the risk. Wildfire burn scars — like the Dixie and Caldor fires that burned across the Sierra in 2021 — could increase runoff because they’re known to have water-repellent waxy soils. Because they are non-vegetated, they are more susceptible to debris flow. During storms, it may affect some downstream watersheds and tributaries, particularly in the Sacramento and San Joaquin river systems.

“We don’t have a big, warm atmospheric river on top of a big snowpack in these areas, and actually, because there were these big wildfires, we’ll be able to do some science tests in real time over the next couple of weeks,” Swain said. .

In 2017, heavy rains and erosion damaged spillways at Oroville Dam, one of California’s largest reservoirs, and more than 100,000 people fled the potential surge of overflowing water. A crisis was averted, but similar bursts of moisture left many on edge about what could be done.

Sean De Guzman, director of water supply forecasting for the California Department of Water Resources, said Oroville — as well as the state’s other major reservoirs, Lake Shasta and Lake Trinity — remain intact. Capacity is limited And there is more space to fill.

“We should see increased volume in the stream, but the reservoirs will not be able to capture anything,” he told reporters on Friday.

Earlier that day, the department’s third snow survey of the year found statewide snowfall at 190% of the average for the date — just below the record set in the winter of 1982-83. In the southern Sierra, snow cover averaged 263%.

The deep snow came as a surprise after fall forecasts leaned toward a drier-than-usual winter. Over the weekend, San Bernardino County officials admitted they were unprepared for the historic storms that swept through the mountains.

As of Monday, tens of thousands of Californians were without power in several snow-covered counties.

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